The narrator goes to visit Usher, because his friend Roderick sent him a letter requesting his company. Roderick wrote he was feeling physically and emotionally ill so the narrator rushes to see him.
social, talkative, loud, outgoing
Any of these words would work as synonyms to gregarious. The antonym clues from the sentences are "quiet and shy". The sentences describe Mateo as gregarious AND the opposite of his brother. Since his brother is described as "quiet and shy", we know that Mateo is the opposite of quiet and shy. Loud and outgoing are the direct opposites of quiet and shy, but there are many other words that can mean the same thing.